Inheritance in Diploids and Haploids: Videos & Practice Problems

Diploid organisms possess two sets of chromosomes, allowing for allele combinations that define traits. Key terms include alleles, dominant and recessive traits, homozygous (same alleles) and heterozygous (different alleles). During meiosis, diploid cells replicate and divide twice to produce haploid gametes, each with one allele per gene. In haploid genetics, wild type alleles are denoted with a plus sign, while mutant alleles lack it. Understanding these concepts is crucial for grasping genetic inheritance and variation.

Diploid Genetics

Hello everyone. In this lesson, we are going to be talking about Diploid Genetics and the genetics of Diploid organisms as a whole. So, diploid genetics is going to be referring to genetics dealing with organisms that have 2 sets of chromosomes or diploid organisms. Understanding the allele combinations that these organisms can have is extremely important to understanding genetics and diploid genetics as a whole. There are many important terms I want you to remember. The important terms that you will learn quickly, because we use them in genetics are going to be terms like alleles, dominant, recessive, homozygous, and heterozygous. Don't worry about memorizing these because you will eventually memorize them due to their frequent use. So, what are alleles? Alleles are going to be variants of a particular trait, but more specifically, alleles are versions of a gene. You can have a gene, let's say it's for eye color, and you can have different versions of that same gene that determine eye color. These are alleles. Alleles are unique versions of the same gene or variants of a particular trait. Diploid organisms are going to have 2 alleles per gene. I'll explain why in just a second.

Now, these alleles or versions of genes can be dominant, and they can be recessive. There are more complicated versions of this. We will also talk about co-dominance, incomplete dominance, sex linkage, and other topics. Don't worry about that as of yet. Right now, I just want you to know the difference between dominant and recessive alleles. Dominant alleles are alleles that, when they are present, are always expressed. A dominant trait is always seen when it is present. A recessive allele is not always expressed when it is present. It is only going to express a recessive trait or phenotype when it is homozygous. So now, let's talk about the difference between homozygous and heterozygous. Homozygous means that there are 2 of the same alleles. Heterozygous means there are 2 different alleles.

Now, let's have a look at an example. These are 2 sets of chromosomes. I know that these chromosomes are from a diploid organism because a diploid organism is going to have a ploidy of 2n. Ploidy is referring to the number of chromosomes or the number of sets of chromosomes that an organism has. Diploid is represented by 2n, and diploid organisms have 2 sets of each chromosome. For example, let's say that an organism has 3 unique chromosomes. If that organism is diploid, it will have 1 and 2 of the first chromosome. This is the number 1 chromosome, and it has 2 versions. And, it is going to have 2 versions of the number 2 chromosome, and it is going to have 2 versions of the number 3 chromosome. We are these types of organisms. We have 2 of each of our chromosomes. If you look at chromosome number 1, your cells will have 2 Chromosome number ones, 2 chromosome number twos, 2 chromosomes number 3, and so on and so forth.

We also have haploid genetics, which we will learn more about later. Haploid genetics means that they only have one of each chromosome. So, one of each chromosome. These cells, let's say, have 3 unique chromosomes. If they have 3 unique chromosomes, what is that going to look like? Well, they're going to have 1 chromosome number 1. They're going to have 1 chromosome number 2. And they are going to have 1 chromosome number 3. The chromosomes are going to be represented by these lines. So haploid genetics means there's only 1 of each chromosome. Diploid genetics means there's 2 of each chromosome. Whenever there are 2 of each chromosome that means that you can have 2 different versions of that same... ...chromosome. This is where different alleles or different versions of genes are going to come in because the same chromosome, chromosome, they have the same genes on them, but they might have different alleles for those genes on them. This is going to be what makes an individual homozygous or heterozygous for a particular gene. So, let's look at this individual right here, and we can see that these are going to be the same chromosomes. So, let's say these are both chromosome number 1, which means they have the same genes. But that does not mean that they have the same alleles. But in this particular case, they do because we can see it says that they're homozygous. So you can see that both of these individuals have this black allele for that particular gene. So they have the same allele. They have identical alleles. So this individual is a homozygous individual right ... ...here. Now, let's look at the same scenario, but now let's look at a heterozygous individual. This individual has 2 unique alleles. So these are different alleles for the same gene. And this can happen in diploid genetics because you have 2 versions of the same chromosome, which are going to hold the same genes. But each of these 2 chromosomes can have unique alleles or unique versions of those genes, which is why you can end up with a heterozygous individual who has different versions of the same genes. So, that is the basis behind diploid genetics.

And, I just wanted you to know that a locus is going to be the specific location of a gene on a chromosome. This is a locus. It is the specific location of that particular gene, and each gene is going to have its own unique locus.

Haploid Genetics

Okay. So now let's briefly talk about haploid genetics. We're just going to briefly do this because, first, it's not as complicated, and second, we're not going to cover haploids as much in this class. But in haploid cells, there's only one allele per gene, which again makes things simpler. Wild type and mutant alleles, in this case, have different symbols than what you may be familiar with. So the Wild type allele is going to look like this. So it's a lowercase letter with a plus sign. Now the plus sign, super important, dictates wild type. You see that? That's the normal version of the wild type. The mutant allele, on the other hand, lacks that plus sign. So you see there's a plus sign here, no plus sign here. That's how you tell the difference between wild type and mutant in haploids. So what happens is that haploids are made in a variety of different ways. One of the ones that's mentioned in your book is the one I'm going to present. And so, this says, we know when opposite mating types fuse. So if you have a wild type and a mutant, so what would that look like? That would look like ^{a+ a}. There's a little cheat for you there. It can actually, so when those two mate together, it actually can create a diploid cell called a myocyte. And this is going to look exactly like this. That's how you're going to write the genotype for that. Now these generally are replicated and then divided to create more haploid cells that then continue the species. Like I said, this is only one way that haploid cells are made, but this is, I guess, a good overview or a good example of what this would look like. So here we start with our haploid cells. We have our one allele, this is the wild type, and therefore, this is the mutant. And how do we know which one is which? Right. That's because of the plus sign. The wild type has the plus sign. When these are mating, they can fuse together, then get this, a bigger haploid cell with both the wild type and the mutant. It replicates, so then you get now four alleles, two wild types and two mutants. And then it can divide. And when it divides, it will create four haploid cells, two wild types, and two mutants. So that's just one example of haploid genetics. But the important thing here is really to understand that haploids have one allele, and their notation is a little bit different with the plus sign and without the plus sign. So, with that, let's now turn the page.

Which of the following describes an acrocentric chromosome?

• A. The p arm is longer than the q arm
• B. The centromere is located at the center of the chromosome
• C. The centromere is located at the end of the chromosome
• D. The p arm and q arm are the same length

Answer: (C)

In diploid organisms there are _______ chromosomal copies. In haploid organisms there is _______ chromosomal copy.

• A. One, two
• B. Two, one
• C. Two, four
• D. Four, two

Answer: (B)

After a diploid cell undergoes meiosis, it divides to produce…

• A. Two diploid cells
• B. Two haploid cells
• C. Four diploid cells
• D. Four haploid cells

Answer: (D)

Here’s what students ask on this topic:

What is the difference between diploid and haploid cells?

Diploid cells contain two sets of chromosomes (2n), meaning they have two alleles for each gene. These cells are typical of somatic (body) cells in organisms. Haploid cells, on the other hand, contain only one set of chromosomes (n), meaning they have one allele per gene. Haploid cells are usually gametes (sperm and egg cells) produced through meiosis. The key difference lies in the number of chromosome sets: diploid cells have two sets, while haploid cells have one.

How do dominant and recessive alleles affect phenotype in diploid organisms?

In diploid organisms, dominant alleles are expressed in the phenotype whenever they are present, even if only one copy is present (heterozygous condition). Recessive alleles, however, are only expressed in the phenotype when both alleles are recessive (homozygous condition). For example, if 'A' is a dominant allele and 'a' is a recessive allele, an organism with genotype 'AA' or 'Aa' will display the dominant trait, while only 'aa' will display the recessive trait.

What is the significance of meiosis in genetic variation?

Meiosis is crucial for genetic variation because it reduces the chromosome number by half, creating haploid gametes, and introduces genetic diversity through processes like crossing over and independent assortment. During meiosis I, homologous chromosomes exchange genetic material (crossing over), and the random assortment of chromosomes into gametes ensures that each gamete is genetically unique. This genetic variation is essential for evolution and adaptation in populations.

What are homozygous and heterozygous genotypes?

A homozygous genotype has two identical alleles for a specific gene (e.g., 'AA' or 'aa'). In contrast, a heterozygous genotype has two different alleles for a specific gene (e.g., 'Aa'). Homozygous individuals will express the same trait for that gene, while heterozygous individuals may express the dominant trait if one allele is dominant.

How are wild type and mutant alleles represented in haploid genetics?

In haploid genetics, wild type alleles are denoted with a lowercase letter followed by a plus sign (e.g., 'a+'), indicating the normal version of the gene. Mutant alleles lack the plus sign (e.g., 'a'). This notation helps distinguish between the normal and altered versions of a gene in haploid organisms, which have only one allele per gene.